In automobile exhaust systems, it is advantageous to introduce air directly into the exhaust manifold or catalytic converter to assist in oxidizing the unburnt gaseous components contained in the exhaust gas. The air is supplied by an air pump, which is driven by the automobile engine and is therefore operable during the operation of the engine. A check valve is usually provided in the air supply conduit leading from the air pump to the catalytic converter or exhaust manifold to protect the air pump from corrosive exhaust gas which may backflow into the air supply conduit.
The presence of air in the air supply conduit downstream of the check valve and upstream of the exhaust manifold or catalytic converter generally provides an air cushion which prevents the high-temperature exhaust gases from coming into contact with the check valve. Nevertheless, when the air pump is not operating, the air cushion dissipates and exhaust gases may seep into the air supply conduit and come into contact with the check valve. These exhaust gases are of relatively high temperature. Therefore, check valves are typically made of heat resistant metals. However, the metal to metal contacts between the parts of these check valves are insufficient to adequately seal the check valve and prevent leakage of air. Furthermore, as the air cushion between the check valve and the source of exhaust gas diminishes, due to leakage in the check valve, exhaust gases are allowed to come into contact with the metal parts of the check valve. As a result, vaporized water and harmful gaseous components contained in the exhaust gas, such as nitrogen oxide and sulfur, condense on the metal parts, forming such corrosive compounds as sulfuric acid. The effect of the corrosive compounds on the metal check valves greatly reduces their ability to prevent a backflow of exhaust gas from reaching the air pump.
One prior device, described in U.S. Pat. No. 3,871,175, attempts to solve this problem by providing two check valves in the air supply conduit: a metal check valve adjacent to the source of high-temperature exhaust gases, and a plastic check valve adjacent to the air pump. Such a system is obviously more costly than a single check valve system, and fails to solve the problem of providing a single check valve which is capable of effectively protecting the air pump from a backflow of exhaust gases.
To overcome the problems caused by corrosion in metal check valves, a plastic check valve is sometimes used. Although constructed of high temperature plastic, the internal parts to these check valves may still be affected by the heat of the exhaust gases. For example, a valve seat subjected to high temperatures may deform, resulting in an incomplete seal between the valve element and the valve seat. As a result, the check valve is unable to prevent the backflow of exhaust gases from reaching the air pump.
Another problem encountered in air supply systems employing check valves is that different makes and models of automobiles require different connections between the check valve and the air supply conduit, the catalytic converter, or the exhaust manifold. In addition, many check valves are manufactured with different lengths and configurations of air supply conduit already attached. Therefore, if a check valve needs to be replaced, the mechanic must return to the manufacturer to obtain an identical check valve assembly, causing substantial delay in returning the automobile to service. Moreover, since some of these existing check valve assemblies are cumbersome, extensive time and effort is usually required to replace a single check valve.